Wednesday, 15 July 2009

at.algebraic topology - Lifting a homeomorphism, always possible?

No. Take any homeomorphism that doesn't preserve the subgroup of $pi_1$ that lift to closed paths in the covering. For example, take the 2:1 covering $S^1to S^1$ take the product with the identity map on $S^1$. Let $h$ be the homeomorphism switching the factors.



In general, I believe a homeomorphism will lift if and only if the associated automorphism of $pi_1$ send the subgroup of the covering to a conjugate.



Another way of saying this is that the category of coverings is equivalent to the category of $pi_1$-sets, and a homeomorphism will lift if the corresponding twist of the $pi_1$-set preserves its isomorphism class.

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